1. Field of the Invention
The present invention relates to a D-A converter, and more particularly, it relates to a current cell matrix DA converter which comprises a cell array arranged in the form of a matrix.
2. Description of the Background Art
FIG. 29 schematically illustrates a matrix of current source cells forming a part of a conventional current cell matrix D-A converter. Throughout the specification, the rows, i.e., the transverse lists of such a matrix are successively numbered starting with 1 from top to bottom. On the other hand, the columns, i.e., the vertical lists of the matrix are successively alphabetized starting from A from left to right. These numbers and symbols are encircled in the drawings. In order to specify one element of such a matrix, the number and symbol indicating its row and column are combined such as "1A" for the cell which is located on the upper left corner of the matrix, for example. Such combinations may also appear in the drawings.
Each of the current source cells 1A, 1B, . . . , 5D, 5E arranged in five rows and five columns comprises a unit current source 20 and a changeover switch 21. Referring to FIG. 29, these reference numerals appear only in relation to the current source cell 1A, for the purpose of simplification.
First ends of the unit current sources 20 are connected along the row direction by row-directionally extending analog source lines (analog ground wires) 101 to 105, and grounded by a column-directionally extending analog ground wire 300. Second ends of the unit current sources 20 are connected to first and second output terminals 31 and 32 through the changeover switches 21 and lead wires 201a to 205a and 201b to 205b respectively. The first and second output terminals 31 and 32 complementarily supply output currents to the exterior.
In the D-A converter having the aforementioned structure, the current source cells 1A, 1B, . . . , 5D, 5E drive the changeover switches 21 provided therein in response to input digital codes, to connect the unit current sources 20 to either one of the first and second output terminals 31 and 32. Thus, currents which are responsive to the input digital codes flow to the first output terminal 31, for D-A conversion.
FIG. 30 is a more simplified version of FIG. 29. In order to clarify the connection between the analog ground wires 300 and 101 to 105 and the lead wires 201a to 205a and 201b to 205b, the unit current sources 20 and the changeover switches 21 are omitted and the contours of the cells are shown in broken lines. In general, the analog ground wires 101 to 105 which are connected by the analog ground wire 300 are further connected to a pad 40 in common, to be grounded.
In the conventional current cell matrix D-A converter, however, the analog ground wires 101 to 105 connecting the first ones of the unit current sources provided in the current source cells are unidirectionally extracted toward the right side in FIGS. 29 and 30, for example, to be connected in common by the analog ground wire 300, and then connected to one or more pads provided in the same direction.
The unit current sources 20 are generally driven by a bias voltage which is supplied to all current source cells in common, to supply currents whose values depend on the bias voltage. When potentials in the analog ground wires 101 to 105 are distributed, therefore, the bias voltage so effectively fluctuates that the currents supplied from the output terminals 31 and 32 to the exterior are not necessarily an integral times those of the unit current sources 20.
For example, FIG. 31 shows an equivalent circuit around the cells 1A to 1E being connected by the analog ground wire 101, which is provided with a distributed resistance shown by ground wire resistances 14a to 14d. Due to such ground wire resistances 14a to 14d, potential distribution which is reduced from the cell 1A toward the cell 1E is caused in the analog ground wire 101.
Therefore, bias conditions for the unit current sources 20 are varied with the cells, and output currents of the unit current sources 20 are varied with the positions of the cells. Namely, current distribution which is increased from the cell 1A toward the cell 1E is caused as conceptually shown in the lowermost stage of FIG. 31.
If the analog ground wire 101 is grounded on its left side, on the other hand, potential distribution which is increased from the cell 1A toward the cell 1E is caused in the analog ground wire 101, while current distribution which is reduced from the cell 1A toward the cell 1E is caused as conceptually shown in the lowermost stage of FIG. 32.
This also applies to the cells which are column-directionally arranged. The analog ground wire 300 connects the analog ground wires 101 to 105 on the right side thereof, while the same is grounded on its lower part through the pad 40. Due to the distributed resistance caused in the analog ground wire 300, therefore, current distribution which is increased from the cell 1A toward the cell 5A is also caused in the column direction.
FIGS. 33A and 33B conceptually illustrates row-directional current distribution and column-directional current distribution, respectively. In practice, however, the unit current sources 20 are discretely provided in the respective cells, and states of current distribution are as shown in FIGS. 34A and 34B in correspondence to FIGS. 33A and 33B, respectively. Referring to FIGS. 341 and 34B, numerals shown in frames represent values of currents actually fed by the unit current sources 20 of the respective cells assuming that the unit current sources 20 feed currents of 5 when the ground wire resistances 14a to 14d are zero.
In the conventional D-A converter, the values of the currents fed by the unit current sources 20 differ from each other both in row and column directions, as hereinabove described. Thus, the actual analog outputs with respect to values represented by input digital codes are deteriorated in linearity with respect to ideal analog outputs.